I have never been impressed or satisfied with my flail mower. As someone showed, anything longer than maybe the top of the fail, gets stripped of leaves, but the stocks remain, even stuff thats like 1/4" in diameter. Wire like grasses, the stems often take 3 or 4 passes, from different directions to really cut.
An important note, mine has "duck feet" teeth; which i had always heard/assumed was half way between side cutter teeth and hammer teeth. I have recently seen some videos that suggest duck feet are a finish cut, low grass only, and the design isnt about heavy duty (vs side cutters), but about air flow, and uplift to give a truly finished cut?
So, maybe thats my issue; but I have never gotten/seen the results other brag about with flails.